1. Field of the Invention
The present invention relates to an aberration adjustment device and methods thereof, an optical pickup, and an optical recording apparatus preferably for recording or reproducing data in DVD or other optical information recording media.
2. Description of the Related Art
Japanese Laid-Open Patent Application No. 2000-348371 discloses a technique of controlling astigmatism in real-time according to a wobbling signal, and adjusting a light spot on an optical disk.
FIG. 1 is a schematic view showing a configuration of an optical disk.
As illustrated in FIG. 1, on an optical disk for use of recording, wobbles 110 and land pre-pits (LPP) 111 are formed to represent synchronization information (synchronization patterns) and address information on the disk. In FIG. 1, the reference 112 represents grooves, 113 represents lands, and 114 represents pits. Because of the presence of the wobbles 110 and land pre-pits (LPP) 111, the structure on the optical disk is microscopically asymmetric in the radial direction. On the other hand, because the track grooves are periodical in units of track pitches, from this point of view, the structure on the optical disk is a symmetric one.
The wobbling signal or the LPP signal is related to the asymmetric aspect of the structure on the optical disk, and the so-called push-pull signal, or other track signals, is related to the symmetric aspect of the structure on the optical disk, which has a relatively low frequency.
FIG. 2 is a plan view showing a state of light received by a light reception element of an optical disk drive.
As illustrated in FIG. 2, the track pattern 103 of reflected light is divided into two parts, that is, a part 104 corresponding to a light reception region A of a light reception element, and a part 105 corresponding to a light reception region B of the light reception element. The wobbling signal or the LPP signal is generated based on a differential signal between a signal corresponding to light of the part 104 and a signal corresponding to light of the part 105. The wobbling signal or the LPP signal is superposed on the push-pull signal related to the track grooves in a high frequency region.
Therefore, if the size of the spot on the recording surface is large in the radial direction (that is, in the direction crossing the tracks), the MTF (Modulation Transfer Function) in this direction declines, and this degrades the ratio C/N (power of carrier/power of noise) of signals obtained from the grooves, that is, the wobbling signal and the LPP signal.
In addition, if astigmatism is present in the light condensed on the recording surface of the optical disk, the front wave of the light becomes a saddle shape, and the spot of such light has different focus positions where the size of the spot is minimum in the direction along which the saddle curves. Therefore, when the light spot is focused in the radial direction, the ratio C/N of signals obtained from the grooves increases.
On the other hand, a sufficiently high MTF is necessary for an information signal along the grooves (that is, in a tangential direction). When astigmatism is present, the light spot seems good if the spot is focused in the tangential direction, but at the same time, the spot size in the radial direction increases, and the signal quality is degraded because of influences of cross-talk between adjacent tracks. Empirically, the position where the amplitude of the RF signal becomes a maximum is the optimum focus position for the information signal. This focus position, however, is different from the focus position where the spot size is in the radial direction.
That is to say, if the astigmatism is present in the light spot, the focus position resulting in an optimum wobbling signal or LPP signal obtained from the grooves is different from the focus position resulting in an optimum information signal.
The astigmatism is generated from optical parts in an optical pickup including the laser diode (LD), and it may change with temperature, hence, troubles due to temperature may occur under different conditions.
Although the technique of controlling astigmatism in real-time according to the wobbling signal and adjusting the light spot on the optical disk is disclosed in the Japanese Laid-Open Patent Application No. 2000-348371, because this technique is based only on the wobbling signal, the aforesaid problems can not be solved.